In variable displacement wobble plate compressors such as are used in motor vehicle air conditioning systems, it is common practice to provide a wobble plate which is connected to pistons by connecting rods in a manner which will produce reciprocation of the pistons within a cylinder head for compressing refrigerant vapors returned to the compressor from an evaporator for cooling air flow into the passenger compartment of the motor vehicle.
Such air conditioning systems have a condenser connected to the discharge of the compressor; a high pressure liquid refrigerant line connected from the condenser to an orifice tube for expanding the liquid refrigerant for flow through the evaporator to cool the air flow thereacross. Refrigerant vapor from the evaporator is directed through an accumulator dehydrator unit in the suction line to the compressor
Such systems operate under various conditions including high vehicle speed operation when the compressor is driven at a constant high speed to produce a maximum displacement of refrigerant gas from the suction line to the discharge line for flow to the condenser. Other operating modes include driving the compressor at reduced speed when the engine is idling during which time the compressor displacement is reduced.
Additionally, when the ambient air is hot and humid a high demand load is placed on the evaporator which can cause the suction pressure of the compressor to increase. When the air is cool, the demand load on the air conditioning system will be low which can cause the suction pressure of the compressor to decrease.
U.S. Pat. No. 4,428,718 discloses a wobble plate whose stroke position is controlled by a combination of suction pressure and discharge pressure in the compressor. In order to adjust the compressor displacement to changes in load on the air conditioning system, the wobble plate has its inclination varied with respect to the drive shaft so as to change the piston stroke in the cylinder head in accordance with the suction pressure of the compressor. The change in inclination is made by changing the pressure in the crankcase of the compressor to reflect the changes in the air conditioning load which are indicated by changes in the suction pressure of the compressor. When a higher suction pressure exists, a pressure sensing valve closes off discharge pressure from the compressor into the crankcase. A pressure balance thereby is produced across the pistons which in turn impose a gas pressure moment on the wobble plate to shift it into a maximum inclination at which the pistons will be fully stroked within the cylinder bores to produce a maximum gas compression displacement from the compressor. Resultant larger quantities of refrigerant flow through the condenser and the evaporator to meet the increased demand placed on the air conditioning system. When the load on the air conditioning system is reduced, the operating condition is reflected as a lower compressor suction pressure. The pressure sensing valve allows discharge pressure into the crankcase so as to increase the pressure in the crankcase. Accordingly, the gas induced moment on the wobble plate is reversed to cause it to assume a destroke position with a small inclination with respect to the drive shaft. Consequently, there is less piston stroke and less refrigerant flow through the air conditioning system.
Other variable capacity compressors are known including U.S. Pat. Nos. 3,861,829; 3,959,983 and 4,073,603 which utilized piston blowby gas flow to the crankcase and control of gas discharge pressure flow to the crankcase to vary the gas moment on the wobble plate.
Still other variable capacity compressors are shown in U.S. Pat. Nos. 4,486,874; 4,737,079 and European Specification EPO 0338762 October, 1989 in which a coil spring is supported between a wobble plate sleeve and the shaft to impose a return force on the wobble plate to shift it from its destroke position to a full stroke position when the gas moment is reversed on the wobble plate. Such coil springs, however, are not arranged to provide a low level spring force between thrust bearings to compensate for shaft end play during assembly of the shaft between a cylinder head and a front head of the compressor. Furthermore, such coil springs are not operative to provide a magnitude of spring bias on a drive shaft to counteract load reversals imposed on the drive shaft during operation at a low air conditioning demand condition.
In the compressor shown in U.S. Pat. No. 4,428,718, the drive shaft is held between the cylinder head and a front head which are joined following assembly of the wobble plate and drive mechanism on the drive shaft and placement of the individual pistons in the bores of the cylinder head. In the past in order to assemble such components clearance was provided between the thrust shoulders of the drive shaft and supporting thrust bearings for the drive shaft such that a small end play remained following assembly. The drive shaft in the '718 compressor may have a load reversal imposed thereon as the shaft rotates during operation of the system only under a low air conditioning demand mode of operation in which the wobble plate is located in its destroke position. Such load reversals act on the shaft to cause it to move fore and aft between the cylinder head and the front head of the compressor causing a chucking noise which is sometimes objectionable.